Tech Bulletin: Troubleshooting Symphony I/O – No Power On

Improperly Seated Cables

If, after connecting AC, the “A” logo doesn’t light, its possible that internal cables have become unseated. To resolve this issue, disconnect the AC cable if connected, remove the top cover and verify that the cables shown below are firmly seated in their mating connector.

1. AC back panel inlet to power supply
2. 2 cables from power supply to Main board
3. Main board to front panel display board

While checking the internal cables, check the 64-pin ribbon cable between the Main board and the I/O Module(s) as described in the document “Tech Bulletin – Symphony I/O Ribbon Cables”.

Once cables are re-seated, re-connect the AC cable and verify that the front panel “A” logo lights up. If it doesn’t, proceed to the next step.

Faulty Power Supply

If, after re-seating cables, the “A” logo still doesn’t light, then either the power supply is faulty or another circuit board is shorted, causing the power supply’s output to be pulled down. The quickest way to determine the faulty part requires a simple voltmeter:

• Set the voltmeter to read DC volts; select a range that includes +24 volts.
• Disconnect the AC cable, then disconnect the 8-pin connector between the power supply and the Main board shown below

• Re-connect the AC cable and CAREFULLY measure for +24 volts DC between any red cable and any black cable on the power supply side of the 8-pin cable.
• If 24 volts isn’t present, then the power supply is faulty and must be replaced.
• If 24 volts is present, then proceed to the next step.

Shorted Circuit Boards

If the power supply measures +24 volts while disconnected, it is most likely operating properly. Now, test the power supply when loaded by all internal circuit boards.

1. Disconnect the AC cable, re-connect the 8-pin cable to the Main board and re-connect the AC cable.
2. Check the “A” logo. If it doesn’t come on with the circuit boards connected, then one of the circuit boards has shorted.

Follow these steps to determine which circuit board has shorted:

• Disconnect the AC cable, disconnect the 64-pin ribbon cable(s) between the IO Module(s) and the Main board (shown below), re-connect the AC cable.

• Check the “A” logo. If it lights when the IO Module is disconnected, then the IO Module is shorted and must be replaced. If 2 IO Modules are installed, connect each one at a time to determine the faulty module.
• If after disconnecting the IO Modules the “A” logo still doesn’t light, disconnect the AC, disconnect the cable between the front panel and the Main board (shown below), and re-connect the AC.

• Since you’ve just disconnected the front panel, the “A” logo can’t light. In order to confirm if the power supply is functioning, measure for 24 v DC at the red & black wires again.
• If the power supply doesn’t measure 24 volts DC, then the Main board is faulty and should be replaced. Here’s the logic we followed to arrive at this conclusion:
• The power supply worked when disconnected:
• The power supply didn’t work when all the circuit boards were connected
• The power supply still didn’t work when the IO modules and the front panel were disconnected but the Main board was connected.
• Conclusion – the Main board is shorted, thus shorting out the power supply’s +24 volt output.

Troubleshooting without a voltmeter

If you don’t have access to a voltmeter, you can still troubleshoot shorted circuit boards by following these steps:

1. Connect AC.
• If the “A” logo doesn’t light up, proceed to step 2.
2. Disconnect AC, disconnect the IO Modules, then re-connect the AC.
• If the “A” logo doesn’t light up, proceed to step 3.
• If the “A” logo does light up, then the IO Module is shorted and must be replaced.
• If 2 IO Modules are installed, connect them one at a time to determine the faulty module.
3. Disconnect AC, replace the power supply with a known good supply, re-connect AC.
• If the “A” logo doesn’t light up, proceed to step 4.
• If the “A” logo does light up, then the initial power supply is faulty and must be replaced.
4. Disconnect AC, replace the Main board with a known good Main board, re-connect AC.
• If the “A” logo does light up, then the Main is shorted and must be replaced. There are no further parts to test!